RFR: 7903665: Function pointer code generation issues

[Maurizio Cimadamore]

This PR fixes two minor issues with function pointer generation:
* static is missing from some nested classes (function pointers)
* function pointers also miss a default constructor

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/212/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=212&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903665
  Stats: 102 lines in 6 files changed: 92 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jextract/pull/212.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/212/head:pull/212

PR: https://git.openjdk.org/jextract/pull/212